How to visualize a value resulted from math node?
Just plug the value directly into compositor output:



Check the Auto Render option in the header to have your composite update.

In the Image Editor see the value with right-clicking on the grey rectangle:



You will see values less than 0 or more than 1 also. No need to scale them.

But you can't see it always, you have to fiddle with the nodes then right-click and check value, can't change node while seeing the output. Blender lacks this functionality.



Theoretically one could make some value visualizer from translating and cropping a picture of numbers with some math involved..or something similar..so here you go:

Image (no need to download its packed in blend):



Blend File with node group:



It will replace your composite with numbers on black background:



This node-group for every decimal of input crops that number from the number-strip (image texture inside the group) and then translates it onto its position.

Due to some rounding artifacts sometimes you might get weird result but its good enough. Its a floating-point limitation.